From GTA BOOM: “There are two daily treasure chests that you can find on the Cayo Perico island while scoping it out. The chest spawn locations are fixed, but they always spawn in completely random order.
One chest always spawns on land, and the other always spawns underwater. Each chest rewards you with GTA$15,000 and 1,000 RP. This means you can get a total of GTA$30,000 and 2,000 RP daily.”
Source: N4G PC All Treasure Chest Locations On The Cayo Perico Island